The present invention relates to a suction box in a paper machine comprising a vacuum chamber, a deck or a number of substantially U-section holders open toward the wire or equivalent web-supporting substrate, a number of ribs arranged in the deck or the holders and loading means for loading the ribs against the wire or equivalent.
In connection with suction boxes in paper machines which are used for numerous functions, such as dewatering a web and guiding a wire, particular problems have been the high consumption of power and the rapid wear of the wires running over the suction box. The ribs or the common deck on a suction box is/are placed against the wire, i.e., in engagement therewith, in which connection, by the effect of the vacuum in the suction box which is applied to the wire through spaces in the deck or between the ribs, a force is produced between the wire and the ribs, which force is divided into a normal force and a friction force. The friction force between the ribs and the wire increases as a function of the vacuum in the suction box and the speed of the wire, and as a result of the friction force, intensive wear of the wire arises. This also increases the requirement of power of the suction box.
In suction box constructions in the prior art, as one solution for these problems, instead of suction boxes, suction rolls have been used, in whose connection, however, the high power loss arising from pumping has been still been problematic. Likewise, the cleaning costs and the high costs of servicing and manufacturing suction rolls have been a drawback of such constructions. Also, it is problematic that when the running speed of the wire becomes high, the consumption of power of the suction roll is increased to a large extent, and also the wear of the wire is increased, for which reasons other solutions are needed for these problems.
An object of the present invention is to provide a solution for eliminating the high consumption of power of a suction box in a paper machine and for preventing the rapid wear of the wires.
It is another object of the present invention to provide a new and improved suction box for a paper machine.
In view of achieving these objects and others, the suction box in accordance with the invention includes a deck or ribs arranged so that tension of a wire running over the deck or ribs is effective to lower the normal force which is produced by the vacuum present in the suction box and which is effective between the wire and the deck or ribs. More particularly, the deck of the suction box is curved or the ribs are arranged to enable a curved run of the wire at least in a running situation in order that the wire tension lowers the normal force which is produced by the vacuum present in the suction box and which is effective between the wire and the deck or ribs.
In the arrangement in accordance with the invention, the friction between the ribs or the common deck of the suction box and the wire is lower or substantially eliminated, in which connection the consumption of power is lowered and the wear of the wires becomes slower, i.e., the wires last longer. In accordance with the invention, in a normal running situation, the common position formed by the ribs of the suction box or the deck of the suction box is curved (concave), so that the wire tension reduces the normal force which is produced by the vacuum and which is present between the wire and the ribs or deck. The curve form of the deck or ribs is produced either by locking the ribs in a curved (concave) form or by making the deck of the suction box or the position of the ribs curved (concave) in the desired manner in advance, i.e., prior to installation in the suction box.
An embodiment of the invention in which the deck of the suction box is fixed and curved (concave) in advance provides a lowering of the friction and thus, the advantages mentioned above. The cost of manufacture of such an embodiment is quite favorable.
With an embodiment of the invention in which the ribs are locked in the desired position and in which a sealing free of contact is achieved, a lower friction force is also achieved, in which case, the requirement of power for the suction box is lowered and the wear of the wire is reduced. With the arrangement in accordance with the invention, the wire can be bent or shaped into a curved form such as a rope curve. According to one preferred additional feature of the invention, it is possible to lock the ribs on the suction box either so that they contact the wire or so that a desired normal force is produced between the rib and the wire, whereby an increased reliability of running is achieved.
Thus, the suction box in a paper machine may comprise means defining a vacuum chamber through which suction is applied, ribs adapted to load a wire and arranged such that spaces are defined between the ribs and communicate with the vacuum chamber, loading means arranged in associated with the ribs to load the ribs toward the wire, and retaining means for retaining the ribs such that while suction is applied through the vacuum chamber and the spaces between the ribs, the ribs cause the wire to curve while running over the ribs such that tension of the wire lowers a normal force produced by the suction. The retaining means may comprise a deck in which the ribs are fixedly mounted. The deck may be stationary and curved.
In the alternative, the retaining means are substantially U-section holders open toward the wire, each holders retaining a respective rib. In this case, locking means are arranged in connection with each rib for locking the same rib in a desired position in the respective holder such that when locking the ribs, the ribs are retracted into the respective holders and locked in a retracted position. The ribs have a bottom surface in opposed relationship to a bottom, interior surface of the respective holder and the loading means, e.g., a loading hose, are arranged between the bottom of each rib and the bottom, interior surface of the respective holder such that the ribs are displaceable by the loading means against the wire. The locking means may comprise a locking rib with slanting faces arranged in a cavity of each holder and a spring member arranged behind the locking rib in the cavity. Sealing means may be arranged at ends of the vacuum chamber for sealing spaces between laterally extending ends of the ribs.
An arrangement for transferring a web from a first wire to a second wire in a paper machine in accordance with the invention includes first guide means for guiding the first wire to support the web over a run thereof, a suction box as described above, second guide means for guiding the second wire to receive the web from the first wire, and a roll arranged in opposed relationship to the suction box such that the first and second wires run between the roll and the suction box. The roll has an open-faced mantle defining a suction chamber and the web is transferred from the first wire to the second during its passage between the suction box and the roll. A last rib in a running direction of the wire may be arranged in an area of a suction sector of the suction chamber or immediately in front of an area of a suction sector of the suction chamber in the running direction of the wire.
In the method for applying suction to a wire in accordance with the invention using the suction box described above, a vacuum is formed in a vacuum chamber of a suction box, ribs are pressed against the wire during formation of the vacuum in the vacuum chamber, and the wire is guided in a curve over the ribs while the vacuum is present in the vacuum chamber such that tension in the wire lowers a normal force produced by the vacuum and which is effective between the wire and the ribs. In one embodiment, each rib is arranged in a U-shaped holder and pressed against the wire by applying pressure to force the ribs outward from a respective holder. The wire is guided in a curve over the ribs by releasing the pressure forcing the ribs outward from the holders such that each rib is freely retractable into the respective holder upon the movement of the wire toward the vacuum chamber, and then locking the ribs in a desired position in relation to the wire once the wire attains the desired curve form. The ribs may be locked in a desired position by applying a force from one side of each holder to press the respective rib against the opposed side of the holder. The ribs may be pressed against the wire in order to increase the reliability of running of the wire. The suction box may be displaced toward or away from the wire to provide desired inlet and outlet angles of the wire in relation to the suction box.
Instead of arranging the ribs in holders, the ribs may be arranged in a stationary deck in a curved form such that spaces are present between the ribs and communicate with the vacuum chamber.
Further advantages and characteristic features of the invention will come out from the following detailed description of the invention.
The invention will be described in detail with reference to some preferred embodiments of the invention illustrated in the figures in the accompanying drawing. However, the invention is not confined to the illustrated embodiments alone.